feat: re-watch new docs_dir when kwelea.toml changes during serve

[engineervix]

Summary

addDirRecursive is called once at startup with the original cfg.Build.DocsDir. If docs_dir is changed in kwelea.toml while kwelea serve is running, the config reloads correctly (fixed in #39) but the fsnotify watcher still watches the old directory tree. Edits under the new docs_dir won't trigger rebuilds until the server restarts.

Current behaviour

1. Start kwelea serve — watcher covers docs/
2. Edit kwelea.toml: docs_dir = "content/docs"
3. Config reloads, next rebuild uses content/docs ✅
4. Edit a file under content/docs/ — no rebuild fires ❌

Expected behaviour

After a docs_dir change is picked up from kwelea.toml, the watcher should stop watching the old tree and start watching the new one.

Proposed solution

In watch, after a successful config reload, compare old and new DocsDir. If changed:

• Remove the old tree from the watcher (or close and recreate the watcher)
• Call addDirRecursive with the new DocsDir

Origin

Surfaced during review of PR #39. Noted by the reviewer as pre-existing behaviour, not introduced by that PR.